A pier is a wharf that projects perpendicularly or obliquely from a shore, serving as a berth for boats to load and unload passengers and cargo or for extending out into the water for other purposes such as fishing, swimming, and recreation. Piers are normally constructed as a pile structure-supported platform using materials selected from steel, coated metal, aluminum, timber, pressure-treated lumber, polypropylene (from milk bottles), fiberglass, and concrete. Piers may be open-deck or housed-over, and are commonly constructed from panels, laths, or sheets assembled and joined together in series on the supporting structure.
Piers and similarly located marine structures are exposed to a wide assortment of hostile environmental conditions that lead to structural failure. Problems with the materials presently used to make piers include catastrophic failure, corrosion, excessive weight, attack by animals (such as worms eating wood), decay, leaching of toxins into the water, non-reparability, cost of manufacture, warping, splitting, brittleness, destruction by the action of waves, the ability of water to transport materials, water damage, UV degradation, chemical attack, color stability, entrapped air pockets, poor resilience (the inability of a material to take an impact without damage), delamination, harm to people, animals and plants in use, water absorption, loss of physical properties, poor stability, limited shapes and designs, costs and methods of assembly, flammability, and overall weather resistance. A material is needed which is reasonably versatile and light in weight, yet highly resistant to the marine (fresh or salt water) environment and having sufficient strength and resilience to function as a dock. The present invention addresses the problems associated with known materials used in dock and pier construction by providing a filled plastic composition having properties uniquely adapted for use in a marine or other outdoor environment.
Filled and unfilled plastic products and methods for their manufacture are well documented, but suffer from a number of problems. Many such known products use a methyl methacrylate polyester resin (acrylic and polyester-based resin) that is brittle. Others use an unsaturated polyester resin which is likewise brittle. Acrylic by itself is very brittle, and can be found in products such as Corian (Du Pont). Combining methyl methacrylate (MMA) with polyester improves UV stability and chemical resistance as compared to unsaturated polyester, but at the cost of making the end product much more brittle. Brittleness in polyester products will depend on the amount of methyl methacrylate (acrylic) used, with larger amounts of acrylic increasing brittleness. A sharp impact or drop to these products will result in a cracked or shattered part, and cracks will propagate within these products. For this reason, such brittle plastics are generally unsuitable for applications such as piers, wherein the structure must absorb occasional impacts (e.g., boats striking the side of the pier) without breaking.
Various inorganic compounds such as aluminum trihydrates (ATH), glass fibers, and glass beads have been used as fillers in products such as the polyesters discussed above. ATH imparts a good flame resistance to polyester, but must be loaded in large amounts to achieve flame resistance (50 to 60% minimum by weight is typical). ATH fillers add weight from 0.6 g/cm.sup.3 to 1.35 g/cm.sup.3 (37.46 lbs/ft.sup.3 to 84.28 lbs/ft.sup.3 for loose-to-packed density) with a specific gravity of 2.42.
Glass fibers impart good strength to a product but increase brittleness and add weight. Glass fibers also have sharp ends which can cause problems to objects that come in contact with them. In the case of a pier in which glass fibers are used, the sharp ends can puncture a foot or hand when a person is touching or walking on the pier. Glass fibers also tend to break off and splinter into a person or object. Overcoating the glass fibers is one of the steps taken to reduce this problem, but adds to cost and labor. Further, all products with such a coating over glass fibers will in time, due to wear, have glass fibers protruding from the product and thus suffer the problems discussed above.
Glass spheres or beads used as a filler provide no strength addition, but provide weight reduction and increased impact resistance. Spheres made of glass add brittleness to the product, and repeated impacts will break the spheres, thereby reducing impact strength with a loss of overall tensile strength. Since the spheres are made of glass and have a rigid nature, violent operations such as mixing the spheres at high speeds, vacuuming to remove air, and spraying the mix at high pressures cause breakage, and the benefits provided by the glass spheres are lost. Similarly, products containing glass fibers or glass microspheres as a filler are not very workable or machinable. Sawing, sanding, nailing, and screw holding are difficult due to the brittle nature of glass. Glass fillers also lack flexibility.
Materials such as recycled polypropylene and polyethylene have many problems when used for the manufacture of piers, benches and other products. These problems include low temperature embrittlement, high temperature softening, flowing and moving, thermal stability (with repeated cold and hot temperature cycling, products will lose physical properties), UV stability (exposure to sun light or UV light will degrade products, resulting in loss of physical properties, and/or loss of color), low strength (unfilled materials are soft, not rigid), and high scratchability due to the soft nature of the material. Additives such as anti-microbial agents added to the products to keep growths from forming on the surface are costly and result in a loss of physical properties.
Processing of filled plastics has been carried out by many methods that are well known. Some methods, such as hand lay-up and injection molding, are used to make piers and other products. These processing methods are expensive and can be labor intensive. Injection molding uses matched metal molds which are expensive. Hand lay-up of materials such as fiberglass is very costly due to the labor needed to make the product, and post-curing the product is needed to attain the desired physical properties. This post-curing step is time and temperature dependent, for example, 2 hours at a temperature of 180.degree. F. or 8 hours at 120.degree. F. is used to cure a marine structure such as a pier or boat haul.
In the casting of polyester marbles and granites, some castings require a gel coat. These gel-coated products are not homogenous, and the gel coat once removed results in a loss of physical properties such as chemical and stain resistance and strength.
In the foregoing methods of processing, entrapped or entrained air is a common problem. In hand lay-up processes, air gaps can reduce the physical properties of the product greatly. In injection molding, air can cause similar problems with physical properties. The present invention provides a composition that avoids the foregoing problems with known materials and processing methods.